1. Field of the Invention
The present invention relates generally to current transfer amplifier, and is directed more particularly to a current transfer amplifier which will remove the non-linear distortion caused by a switch.
2. Description of the Prior Art
A prior art audio amplifier is of a so-called constant voltage transfer type which uses various signal voltage sources such as a phono-cartridge, tape recorder, tuner or the like and drives the speaker thereof in the form of a voltage, so that the switching and transfer of the signal are all carried out in the voltage mode.
The audio amplifier of the constant voltage type, in which the signal switching (processing) and signal tranfer are carried in the voltage mode, can not be free from the following various problems. That is, before a signal is transmitted or transferred to a power amplifier, the signal passes through many non-linear elements such as a switch and so on. In this case, however, since all the non-linear elements are deemed as a series impedance, the input signal to the power amplifier is determined as the voltage dividing ratio of the impedance. While, when viewed from the input terminal of the power amplifier, a closed loop which terminates at the output impedance is formed. Therefore, an electromotive force is generated by the magnetic flux which passes through the loop area and hence hum and flux at the output stage of the speaker are easily picked up. Further, since a closed loop is formed when viewed from a load terminal, a ground impedance is inserted as a series impedance. Therefore, a return current must be returned to a constant voltage source. It is rather difficult that, in order to reduce the affect by the return current, the ground potentials at a pre-amplifier stage and a power amplifier stage are made coincident. For this reason, there are necessary various counter measures, for example, a bus bar for the ground is used to reduce the series impedance or a shield wire is used for the same purpose. Further, since in the constant voltage type a variable resistor for volume adjustment is of the resistance value dividing type, when the variable resistor for volume adjustment is adjusted as, for example, about 1/2 in volume i.e. volume is reduced by -6 dB, a noise becomes maximum, namely the S/N ratio becomes worst. Also, even upon a normal using state (the variable resistor for volume adjustment is reduced further from 1/2), the S/N ratio is deteriorated by about 10 to 20 dB. Thus, there appears such a problem that very unnatural sense is caused in view of hearing.
As described above, the audio amplifier of the constant voltage type proposes various problems. In fact, as the audio amplifier is improved in property, non-linear distortions by the change-over switch, materials of wires and so on become problems. At present, the property of high accuracy or distortion ratio of, for example, about 0.003% is required, so that the non-linear distortion by the changeover switch or material of wires (or relay and so on) can not be neglected.